1. Field of the Invention
The present invention relates to methods and devices for spectral analysis, and in particular to a method and circuit for determining the presence or absence of at least one frequency of known value in an input signal comprised of several different frequencies.
2. Description of the Prior Art
The necessity of determining the presence or absence of specific frequencies with a known value in an input signal comprised of several frequencies arises in decoding the program type recognition signal in a two-carrier television sound system. Details of such a two-carrier television sound system are described in CCIR Recommendation 599, Report 795-1, "Directivity of Antennas for the Reception of Sound Broadcasting in Band 8 (VHF)," 1978-1982, at pages 205-213. As described therein, program type recognition in the case of a two-carrier television sound system proceeds with the use of an amplitude-modulated pilot carrier tone. The frequency of the pilot carrier tone is equal to 3.5 times the video line frequency f.sub.H. For the stereo and two tone operating modes, an identifying tone is modulated on the pilot carrier tone. The possible operating modes are schematically represented in FIGS. 1A-1D in the form of a frequency spectrum.
The stereo operating mode is shown in FIG. 1A in which an identifying tone with the frequency of 15,625/133 Hz is modulated, by amplitude modulation, onto a pilot tone carrier, whose frequency is references f.sub.p, which has a frequency of 3.5.times.15,625 Hz. The value of 15,625 Hz is the video line frequency f.sub.H. In FIG. 1A, the side bands are identified at f.sub.+s and f.sub.-s, respectively.
The two tone operating mode is shown in FIG. 1B wherein an identifying tone with a frequency of 15,625/57 Hz is modulated onto the pilot carrier tone having a frequency referenced f.sub.p. The side bands in this mode are identified at f.sub.+z and f.sub.-z, respectively.
The mono operating mode is shown in FIG. 1C in a two-carrier television sound system in which only the pilot carrier tone having a frequency f.sub.p is present, and on which no identifying tone is modulated.
Lastly the conventional mono operating mode is shown in FIG. 1D in the case of present-day conventional television transmission in which the pilot carrier tone is not present.
The modulation degree in the case of the operating modes shown in FIGS. 1A and 1B is 50%.
Transmission of the above-explained amplitude modulated frequency proceeds from a television transmitter such that the amplitude-modulated pilot carrier tone is modulated, by frequency modulation, on a carrier in the megahertz range. In the television receiver, an initial frequency demodulation and a subsequent amplitude demodulation are conducted for obtaining the identifying tones. Evaluation of the information identifying the operating modes mono, two-tone, and stereo can proceed via the upper or lower side band. Evaluation of either side band is equivalent.
In conventional systems, decoding of the program type recognition in a two carrier television sound system is conducted by means of synchronous or envelope detection of the modulated pilot carrier tone with a subsequent evaluation of the demodualted identifying tone.
In transmission of television information, due to cross-modulation effects, interfering frequencies from the video component of the television information may result which are identical to the frequency of the pilot carrier tone. These interfering frequencies may be so strong that corresponding fluctuations of the pilot carrier tone amplitude, and hence undesired interference frequencies, occur after the amplitude demodulation.